Low dielectric composite fiber and fabric

ABSTRACT

A composite fiber prepared by wrapping a core of polyethylene filaments having a dielectric of less than 3.0 with a direct sized quartz fiber to provide a composite fiber and fabric woven therefrom with improved high temperature strength and low dielectric constant.

FIELD OF THE INVENTION

This invention relates to reinforcing fabrics which are to be coatedwith thermosetting resins and, in particular, relates to low dielectricfabrics and the fibers from which they are woven to form the substratefor radomes and other shaped, protective structures that have minimuminterference with the transmission and reception of electromagneticsignals.

BACKGROUND OF THE INVENTION

The use of fabrics woven from polymeric fibers and fiberglass toreinforce structural members is well known and the use of thesematerials has proven satisfactory over a number of years. However, in avariety of specialty applications which include protective enclosuresfor electronic equipment that receives and transmits electromagneticsignals, it is quite important that the protective enclosure interfereswith the transmission to only the minimal extent possible. Thus, it is aprimary object of the present invention to provide a reinforcing fabricwith a low dielectric constant that will be particularly suitable forreinforcing such structures.

Another important factor in developing reinforcing fabrics is to weavethe fabric in a manner so that the fabric can be stretched to conform tothe desired shape of the article to be formed without wrinkles, bulges,or distortions. Such fabrics are sold under the brand name, "ConformFabrics®". Conform Fabrics and fabrics of other weave patterns aredisclosed in U.S. Pat. No. 5,102,725 which issued on Apr. 7, 1992 toMartha Knox et al and is incorporated herein by reference.

Certain polymeric filaments that can be spun and woven into fabrics havelow dielectric constants but tend to soften and weaken when a heatedmolten thermosetting polymeric material is applied to the fabric as acoating; or, if the coating is applied by immersing the fabric in a bathand then cured by heating at an elevated temperature for an extendedperiod, the curing temperature may be in the range of the glasstransition temperature of the polymeric filaments. Thus, it is anotherobject of the present invention to provide a low dielectric fabric withimproved strength to withstand the application of heated coatings andcuring temperature.

In the past, reinforcing of an aramid fiber by twisting with fiberglasshas been proposed as disclosed in U.S. Pat. No. 4,528,223 which issuedon Jul. 9, 1985 to Kumazawa et al. However, aramid polymers tend to havea relatively high dielectric constant and tend to absorb moisture whichfurther raises the dielectric constant. Thus, it is yet another objectof the present invention to provide a fabric with added strength, lowerdielectric constant, and improved bonding to applied coatings than thatwhich can be provided by such prior art fibers and fabrics.

Also, available and useful polyaramid fibers such as those sold underthe Kevlar® brand and polyethylene fibers sold under the Spectra® brandmust be plasma etched in order to increase the bondability of theirsurfaces to a satisfactory level. Furthermore, the plasma etching effecthas a very limited shelf life, i.e., the surface impregnation andbonding improvement are time dependent so that the fiber and/or fabricso treated must be woven and coated without prolonged delay or storage.Accordingly, it is another object of the present invention to avoid thenecessity of plasma etching.

These and other objects of the present invention will become generallyapparent to those skilled in the art from the following Summary of theInvention and Detailed Description.

SUMMARY OF THE INVENTION

It has been surprisingly discovered that by selecting a core ofpolymeric filaments having a dielectric constant of less than or equalto the dielectric constant of quartz fiber and wrapping them with adirect sized quartz fiber that a composite fiber can be produced whichhas high strength at the curing temperatures of thermosetting coatingmaterials, low dielectric constant, superior bonding properties and doesnot need to be plasma etched. The composite fiber may be used in weavinga fabric particularly, a reinforcing fabric, or the fibers may bewrapped around a mold to form a reinforcing structure.

It has also been found that a core of polymeric filaments, particularly,high molecular weight polyethylene filaments that have a dielectricconstant of about 2.2 or less, when wrapped with a direct sized quartzfiber produce a yarn from which especially suitable reinforcing fabriccan be woven. A combined dielectric constant of less than 2.6 can beachieved. While polyethylene is a preferred polymer, the inventionincludes fibers and filaments of organic carbon based polymers ingeneral.

In one aspect, the present invention is a method of providing a core offilaments which have been selected so that said filaments have acombined or composite dielectric constant of less than about 3.78, thedielectric constant of quartz, and then wrapping the core with a directsized quartz fiber in a sufficient number of turns per unit length witha gap between turns so that the volume of quartz has not raised thedielectric constant of the resulting composite fiber above about 3.78yet the desired strength is attained.

In another aspect, the present invention is a fabric woven from theaforementioned composite fiber which is woven in a suitable weave asmentioned herein above. The yarn used in this fabric is also suitablefor impregnation for filament winding of composite structures.

In still another aspect, the invention includes a wrapped fiber whereinthe wrapping allows a gap so that when a fabric is woven from the fabricand subsequently heated, the polymeric material exposed by the gap willundergo limited flow and melt bond to adjacent polymeric fibers.

In yet another aspect, the present invention is a fabric that can becoated with a thermosetting resin to form a shaped article.

The present invention will be better understood by reference to theDefinitions, Description of Drawings, and Detailed Description whichfollow.

DEFINITIONS

As used herein the following definitions may be referred to by way ofexplanation of terms but are not limiting on the scope of their meaning:

"Dielectric" has its usual definition of a material that is so weaklyconductive that different parts of a sheet of it can hold differentelectrical charges and generally is considered to have a conductivity ofless than 10⁻⁶ siemens per centimeter. In common usage it is oftenreferred to in a relative manner and the "dielectric constant" providesthe relative comparison.

"Dielectric constant" is based on a vacuum having a constant of 1.0000and the ASTM Method as reported in D150 and D1531 is used to determinethe constant for material.

"Weave patterns" referred to herein include those that are common to thetextile industry. Quite useful patterns are described in theaforementioned patent to Knox et al. In particular, the weaves known aseight harness satin and those of the Conform Fabrics® are preferred.

"Polymer" as used herein, includes homopolymers, copolymers,terpolymers, graph, block and higher polymers. Fibers of the ethylenepolymers are particularly suitable. Especially preferred is the extendedchain, ultra-high molecular weight polyethylene sold under the brandname Spectra®.

"Polymeric coating" includes thermosetting coating materials which maybe applied by spraying, rolling, extruding, or immersion to areinforcing fabric substrate and include polyesters, transfer moldingresins, cyanate esters, and epoxies. For the present invention, in animmersion coating process, the total finished weight of a fabric mayhave 2% to 60% of the weight represented by coating material which issometimes also referred to as the "coating resin".

"Sized" and "Direct Sized" refers to surface modification of the fibers.Particularly, the quartz fiber referred to herein, preferably includesthe application of a coupling agent which is capable of reacting withboth the quartz and the coating resins so that a strong bond can beformed therewith. Generally preferred are the silane coupling agents andthe use of these or similar agents is referred to as "direct" sizing.

"Quartz"as used herein and "quartz fiber" mean fibers produced fromfused quartz of high purity, namely about 99.95% SiO₂ and, because ofits purity and amorphous structure, it is distinct in properties andperformance from glass, glass fibers, or other silica containingproducts.

DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to theaccompanying drawings that form a part of this specification and areexamples of embodiments of the invention but are not limiting to thescope thereof:

FIG. 1 is a schematic representation of a low dielectric polymeric corewrapped by sized quartz fibers;

FIG. 2 is a representation of a coated fabric with a breakaway sectionshowing warp yarns that comprise the fiber shown in FIG. 1; and,

FIG. 3 is a representation of a coated fabric wherein the sectionalbreakaway view shows both warp and fill yarns which comprise the fiberof FIG. 1.

DETAILED DESCRIPTION

Turning now to FIG. 1, core fiber 1 is shown which represents a bundleof polymeric filaments 2 (not shown individually) and these have beenselected because they have a dielectric constant of less than about 3.0.Such filaments include those of olefin polymers such as propylene andethylene. Especially preferred are the extended chain, ultra-highmolecular weight polyethylenes sold under the Sprectra® trademark. Thelinear density, D, of the core preferably ranges from about 1,800 to30,000 yards per pound and is schematically represented as a diameter inFIG. 1.

The core is wrapped by quartz fiber 3 at a wrap angle φ which can varyas preferred from as low as a few degrees up to and approaching 90°. Thenumber of turns, "n", per inch, represented by length, "L", can varyfrom one to about 20 with a preferred range being about 6 to 12. Thequartz fiber is direct sized with a coupling agent which preferably is asilane coupling agent which is well known in the art. The fiber lineardensity, "d", may range from 100 to 5,000 denier. Wrapping is apreferred feature of the invention and is distinguished from a commontwist so that the core comprises the major linear volume proportion ofthe composite fiber. Thus, the gap or spacing "s" will vary according tothe denier of the quartz fiber and the turns, "n".

The method of winding the wrap around the core can include such methodsas a hand lay-up or wind-up method, or a cabling method. A typicalmachine for producing such a wrap is a ICBT Cabler which is well knownto those skilled in the art. Other suitable processes are also wellknown to those skilled in the art. After wrapping, the low dielectriccomposite fiber 5 of the invention is formed.

Turning now to FIG. 2, fabric 10 is shown in the breakout section whichhas been woven with fibers 5 serving as the warp yarn. An eight harnesssatin fabric weave is preferred. The fabric has been coated withthermosetting polymeric material 20. This polymer may be applied byspraying, hot melt, or by immersing in a coating bath. Thermosettingresins are used as a protective coating and as they cure and "set up"they tend to preserve the shape and dimensional stability of theresulting shaped structure. The important consideration here is that theuseful thermosetting resins normally will be cured in the range of 250°F. to 350° F. This temperature is above the glass transition temperatureof polymers such as polyethylene so that the strength added by thequartz wrapping of the core fiber is essential. In the preferred processthe fabric is dipped or immersed into a coating solution and will pickup a loading of 30% to 40% of its weight from a thermosetting polymerssuch as cyanate esters or epoxies. The usual curing time runs about twohours. The coating solutions and baths are well known to those skilledin the art.

The particularly preferred core fiber material provided by filaments ofthe polyethylene sold under the brand name of Spectra® by AlliedChemical Company has a dielectric constant of about 2.2. Quartz fiberscan be obtained from QPC, Inc. and these fibers have a dielectricconstant of about 3.78.

PREFERRED EMBODIMENT

In a preferred embodiment which is the best mode of the invention, acore fiber of filaments of the Spectra® polyethylene was prepared andwrapped with quartz fiber at 15 turns per inch using an ICBT Cabler. Thecore fiber unit weight or linear density was about 5,000 yards per poundand the quartz fiber was about 300 denier. The quartz fiber wasidentified as 300 2/0 from QPC. On a mass per length basis, these unitscovert to about 0.042 grams of quartz per meter and about 0.102 grams ofpolyethylene per meter. At the estimated average diameter of the core,15 turns per inch approximates an additional quartz fiber in each meterso that on a wrapped basis there is a total of 0.084 grams of quartz permeter wrapped onto 0.102 grams of polyethylene. Since the dielectricconstant is proportional to volume, the volumes, based on the respectivedensities, are 0.93 for polyethylene and 2.2 for quartz. The resultingvolume per meter is 0.032 cm³ for quartz and 0.109 cm³ for polyethylene.Thus, quartz is 22% and the polyethylene is 78% of this composite. Thedielectric constant for quartz is 3.78 and for polyethylene is 2.2; and,on a volume ratio basis, the composite dielectric constant is calculatedto be 2.47.

The so prepared fiber is used as the warp and/or fill yarn to weave aneight harness satin fabric which is dipped in a coating bath and thenremoved after it has picked up about 35% of its weight of cynate esterresin, then solvents are removed by heat, thus producing a reinforced,protective covering material suitable for manufacturing radome coversand similar equipment.

Various resin coating systems can be used with the fabrics of thisinvention which provide a low dielectric constant and improved strength.Such coated fabrics can be shaped into may different articles where alow dielectric constant is required.

While we have shown and described particular embodiments of theinvention, modifications and variations thereof will occur to thoseskilled in the art and subsequently observe the invention or read thisspecification. We wish it to be understood therefore that the claimsbelow are intended to cover such modifications and variations which arewithin the scope and spirit of the invention.

What is claimed is:
 1. A low dielectric, composite fiber suitable forweaving into a reinforcing fabric comprising:a) a core of polymericfilaments having a dielectric constant of less than 3.0; b) a directsized quartz fiber wrapped around said core; and, c) said compositehaving a dielectric constant of less than about 3.0.
 2. The compositefiber of claim 1 wherein said polymeric filaments are selected from thegroup consisting of polyethylene filaments.
 3. The composite fiber ofclaim 2 wherein the filaments are extended chain, ultra-high molecularweight polyethylene.
 4. The composite fiber of claim 1 wherein thequartz fiber comprises quartz filaments that are directly sized with asilane coupling agent.
 5. The composite fiber of claim 4 wherein saidquartz fiber is wrapped between 1.0 and 20.0 turns per inch around saidcore.
 6. The composite fiber of claim 1 wherein said core fiber rangesfrom 1,800 to 30,000 yards per pound.
 7. The composite fiber of claim 5wherein the quartz fiber ranges from 100 to 5,000 in denier.
 8. Thecomposite fiber of claim 1 wherein the dielectric constant is less thanabout 2.6.
 9. A woven fabric comprising warp and/or fill threads of thecomposite fiber of claim
 1. 10. A woven fabric comprising warp and fillthreads of the composite fiber of claim
 1. 11. The woven fabric of claim9 wherein said fabric is coated with a polymeric coating material toform a shaped article.
 12. The composite fiber of claim 1 wherein thenumber of turns per inch of the quartz fiber is selected so that thedielectric constant of said composite fiber is below 3.0.
 13. Thecomposite fiber of claim 1 wherein the core filaments are polyethylenehaving a linear density of about 5,000 yards per pound and the core hasbeen wrapped at about 15 turns per inch with a sized quartz fiber ofabout 375 denier.
 14. A low dielectric, reinforcing fabric comprising:a)warp yarns comprising a core of polymeric filaments having a dielectricconstant of less than 3.0, said core being wrapped by direct sizedquartz fibers; and, b) fill yarns selected from the group consisting ofthe yarn of subparagraph a), yarns having cores of ethylene polymers,and yarns having cores of aramid polymers wherein the selected yarns arewrapped with direct sized quartz fibers.
 15. The fabric of claim 14wherein said fabric is woven in a satin weave.
 16. The fabric of claim14 wherein the fabric is woven in a manner such that the fabric can bestretched to conform to a desired shape without wrinkles, bulges ordistortions.
 17. The fabric of claim 14 wherein the core filaments areselected from the group consisting of ethylene polymers.
 18. The fabricof claim 14 including a polymeric coating thereon suitable for forming ashaped article.
 19. A method of preparing a low dielectric compositefiber which is suitable for use in weaving a reinforcing fabriccomprising the steps of:a) providing a core fiber of polymeric filamentsselected from the group of filaments having a dielectric constant ofless than 3.0.; and, b) wrapping said core with a direct sized quartzfiber so that the number of wrapping turns per unit length of core fiberminimizes the gap between turns, but does not raise the resultingdielectric constant of said composite to above 3.0.
 20. The method ofclaim 19 wherein the polymeric filaments comprise extended chain,ultra-high molecular weight polyethylene.